As environmental concerns have led to increasingly strict regulation of engine emissions by governmental agencies, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important. Current indications are that this trend will continue.
Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. In the past, the emission levels of US diesel engines have been regulated according to the EPA using the Federal Test Procedure (FTP) cycle, with a subset of more restrictive emission standards for California via the California Air Resources Board (CARB). For example, the Tier II emission standards, which are being considered for 2004, are 50% lower than the Tier I standards. Car and light truck emissions are measured over the FTP 75 test and expressed in gm/ml.
Regulatory agencies continue to propose and apply stricter emission standards. For example, proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi NOx and 0.08 gm/ml particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and ULEVs in California would have to meet a 0.05 gm/ml NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi would also have to be met. The EPA has also proposed tighter regulations for off-road diesel engines, requiring them to emit 90% less particulate matter and nitrogen oxides, by 2014 than they do today.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping, by themselves will not be able to achieve these low emission levels required by the standard. Aftertreatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine.
Some promising aftertreatment technologies to meet future NOx emission standards include lean NOx catalysts, Selective Catalytic Reduction (SCR) catalysts, and Plasma Assisted Catalytic Reduction (PACR). Current lean NOx catalyst technologies will result in the reduction of engine out NOx emissions in the range of 10 to 30 percent for typical conditions. Although a promising technology, SCR catalyst systems require an additional reducing agent (aqueous urea) that must be stored in a separate tank, which opens issues of effective temperature range of storage (to eliminate freezing) as well as distribution systems that must be constructed for practical use of this technology. PACR is similar to lean NOx in terms of reduction efficiency but is more expensive due to the required plasma generator. These technologies, therefore, have limitations that may prevent their use in achieving the new emissions requirements.
Therefore, there is a need for an engine aftertreatment system that provides a source of heat and reductants to regenerate NOx absorbents and does not result in a significant fuel penalty. The present invention is directed toward meeting this need.